This invention relates to PEM fuel cells and more particularly to the reactant flow fields therefor.
Fuel cells have been proposed as a power source for many applications. One such fuel cell is the PEM (i.e., proton exchange membrane) fuel cell. PEM fuel cells are well known in the art and include in each cell thereof a so-called xe2x80x9cmembrane-electrode-assemblyxe2x80x9d (hereafter MEA) comprising a thin (i.e., ca. 0.0015-0.007 inch), proton-conductive, polymeric, membrane-electrolyte having an anode electrode film (i.e., ca. 0.002 inch) formed on one face thereof, and a cathode electrode film (i.e., ca. 0.002 inch) formed on the opposite face thereof. Such membrane-electrolytes are well known in the art and are described in such U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the Journal of Power Sources, Volume 29 (1990) pages 367-387, inter alia. In general, such membrane-electrolytes are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION(trademark) available from the E.I. DuPont de Nemours and Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material (e.g., NAFION(trademark)) intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder. One such MEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec. 21, 1993, and assigned to the assignee of the present invention.
The MEA is sandwiched between sheets of porous, gas permeable, conductive material, known as a xe2x80x9cdiffusion layerxe2x80x9d, which press against the anode and cathode faces of the MEA and serve as (1) the primary current collectors for the anode and cathode, and (2) mechanical support for the MEA. Suitable such primary current collector sheets comprise carbon or graphite paper or cloth, fine mesh noble metal screen, and the like, through which the gas can move to contact the MEA underlying the lands, as is well known in the art.
The thusly formed sandwich is pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors and conducting current between adjacent cells (i.e., in the case of bipolar plates) internally of the stack, and externally of the stack in the case of monopolar plates at the ends of the stack. The secondary current collecting plates each contain at least one so-called xe2x80x9cflow fieldxe2x80x9d that distributes the fuel cell""s gaseous reactants (e.g., H2 and O2/air) over the surfaces of the anode and cathode. The flow field includes a plurality of lands which engage the primary current collector and define therebetween a plurality of flow channels through which the gaseous reactants flow between a supply manifold at one end of the channel and an exhaust manifold at the other end of the channel. Serpentine flow channels are known and connect the supply and exhaust manifolds only after having made a number of hairpin turns and switch backs such that each leg of the serpentine flow channel borders at least one other leg of the same serpentine flow channel (see U.S. Pat. No. 5,108,849). Serpentine channels are advantageous in that they permit some gas flow between adjacent legs of the same channel; via the diffusion layer. In this regard, gas can flow from an upstream portion of the channel to a downstream portion of the channel where gas pressure is lower (i.e. due to the pressure drop down the length of the channel), by flowing through the diffusion layer over the land that separates the upstream leg from the downstream leg portion of the flow channel.
The pressure drop between the supply manifold and the exhaust manifold is of considerable importance in designing a fuel cell. One of the ways of providing a desirable pressure drop is to vary the length of the flow channels extending between the supply and exhaust manifolds. Serpentine channels used heretofore (e.g., see U.S. Pat. No. 5,776,624) limit design flexibility in that such channels require an odd number of legs (e.g. 3,5,7 etc) that extend most of the distance between the manifolds. Hence the length of each channel is in large part determined by the distance between the manifolds.
The present invention overcomes the aforesaid problem by providing serpentine flow channels whose length can be varied, essentially at will, without regard for the distance between the manifolds and without having any unused space in the flow field. More specifically, the present invention is an improvement to PEM fuel cells of the type discussed above which comprises: a proton exchange membrane having opposing cathode and anode faces on opposite sides thereof; a gas-permeable, electrically conductive cathode current collector engaging the cathode face; a gas permeable electrically conductive anode current collector engaging the anode face; and a current-collecting plate engaging at least one of the gas-permeable collectors and defining a gas flow field that confronts that gas-permeable collector. The flow field comprises a plurality of lands that engage the current collector and define a plurality of substantially equal-length serpentine gas flow channels, each of which has: an inlet leg for receiving gas from a supply manifold that is common to all of the flow channels; an exit leg for discharging said gas into an exhaust manifold that is common to all of the flow channels; and at least one medial leg that lies intermediate the inlet and exit legs. The inlet, exit and medial legs for each channel border at least one other leg of the same channel. In accordance with the present invention: one of the inlet and exit legs of each channel extends for a first length from its associated supply or exhaust manifold in the direction of the other manifold; the other of the inlet and exit legs extends in the same general direction as the one inlet and exit leg for a second length that is less than said first length; the medial leg extends in the same general direction as the inlet and exit legs for a third length that is less than the second length and is defined by a land which is spaced from, and substantially aligned lengthwise with, a similarly situated medial leg of an adjacent flow channel; and a hairpin curve in the channel at each end of the medial leg connects the medial leg to adjacent legs of the same channel. Each serpentine channel may include one or more medial legs to vary its length. In one embodiment of the invention, the length of the medial leg is less than about one half that of the longest of the inlet and exit legs. In another embodiment, the medial leg is less than about one third the length of the longest of the inlet and exit legs. In still another embodiment, the medial leg is less than about one quarter the length of the longest of the inlet and exit legs. In accordance with another embodiment yet, the supply and exhaust manifolds lie at opposite ends of the flow field and one of the inlet and exit legs extends for nearly the entire length of the flow field between the manifolds. Preferably, the inlet legs of adjacent channels border each other and the exit legs of adjacent channels border each other, but the inlet legs do not border the exit legs for the same reasons as set forth in copending U.S. patent application Ser. No. 09/016,127 filed Jan. 30, 1998 in name of Jeffrey Rock, and assigned to the assignee of the present application.